The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a system and method for detecting and tracking positional changes in a reference structure using a linear phase shift technique to eliminate or significantly reduce the effects of motion artifacts in an MR image.
Respiratory motion artifacts degrade image quality in non-breath-held MR scans. That is, MR scans where the patient is either allowed to breath freely, breaths inadvertently, or in MR studies that require scan times in excess of a typical patient's ability to hold their breath. In these cases, some technique other than simply having the patient hold their breath, must be used to minimize respiratory motion artifacts.
A class of techniques, known as navigator-type sequences use MR to periodically image a two-dimensional column of spins that include the diaphragm. By detecting changes in the diaphragm position, data acquisition can be synchronized to a common position in the respiratory cycle. In this manner, MR data acquisition is gated to a specific position of the diaphragm, and by implication, to a specific position of the internal organs in the thoracic and abdominal cavities.
Prior art methods of detecting such positional changes have relied upon the use of cross-correlation or least-squares algorithms. Such algorithms are computationally intensive since they require cross-correlation or least-squares determination against a reference profile for a series of different pixel shifts in the sample viewed. The spatial shift that corresponds to the greatest value of the cross-correlation or the minimum value of the least-squares difference will yield the position of the diaphragm relative to the reference position, but only in terms of whole pixels. With such methods, the reference profile or position must always be manually selected before the start of data acquisition by an MR scan operator.
An alternate prior art method to using navigators is to use the phase of the echo peak, or the center of the k-space phase, as an indication of the relative displacement of the reference object. Although this has been used to correct for motion-related artifacts in neuro-functional imaging studies, it is not suitable for monitoring the displacement of the diaphragm because the phase of the echo peak includes the phase of all spins in the image field-of-view and is unable to extract the displacement of a single reference structure. Further, such a method cannot monitor diaphragmatic motion where a projection profile in the superior-inferior orientation includes moving structures (liver, stomach, etc.) and non-moving structures (lung field, shoulder). This method also requires manual input of an initial positional selection by an MRI operator.
Therefore, it would be desirable to have a system and method for detecting and tracking positional changes in a reference structure that is computationally efficient, is not reliant on operator input or influence, or on pixel size, and eliminates the need to require a patient to breath-hold, thereby eliminating an additional patient stress factor during a magnetic resonance imaging procedure.